1. Field of the Invention
The present invention relates to readout signals with an increased signal to noise ratio in an optical information reproducing apparatus.
2. Description of the Related Art
For optical disks, Blu-ray Disc™ using a blue-violet semiconductor laser and a high numerical aperture (NA) objective lens having a numerical aperture of 0.85 becomes commercially available, reaching almost the limit to the resolution of optical systems. In order to aim for a further increase in capacity, it can be considered that an increase in the number of recording layers is most likely in future. For years, disks according to Blu-ray Disc Rewritable Dual Layer having a layer spacing of 25 μm between recording layers are commercially available, which are used for image storage media of recorders and data storage media of personal computers.
In such multilayer optical disks, because it is necessary to provide almost the same light quantity that is detected from each of individual recording layers, the reflectance of a certain recording layer is inevitably made small in association with an increase in the number of recording layers. Because of this, there is a problem in that the signal-to-noise ratio of the readout signal from each of the individual recording layers is decreased.
In addition, in the case in which a laser beam is directed to a multilayer optical disk, this causes plural layers to be irradiated at the same time. Thus, the signal-to-noise ratio of the readout-signal is also reduced due to the stray light reflected off a recording layer other than the target layer from which information is reproduced, i.e., due to interlayer crosstalk.
However, for optical disks, the data transfer rate is still increasing because of the necessity of an increased dubbing speed for video or the like while the capacity is increasing, so that it becomes difficult to sufficiently ensure the signal-to-noise ratio of the readout signal as matters stand. Therefore, in order to advance increases in the number of recording layers and in the data transfer rate in future at the same time, it is necessary to provide the readout signal with an increased signal to noise ratio.
Techniques relating to providing the readout signal of the optical disk with an increased signal to noise ratio are described in Japanese Patent Application Laid-Open Publication Nos. Hei 5-342678 and Hei 6-223433. They all relate to aiming for providing the readout signal of the magneto-optical disk with an increased signal to noise ratio by amplifying the amplitude of a weak signal, in which a light beam from a semiconductor laser is divided into light beams before directed to an optical disk and the divided light beam not to be directed to the optical disk is combined with the divided light beam reflected off the optical disk for interference to increase the quantity of the light beam not to be directed to the optical disk. In differential detection between transmitted light and reflected light at a polarizing beam splitter that is conventionally used in the signal detection of the magneto-optical disk, inherently, an original incident polarized component is caused to interfere with a polarized component, which is produced due to rotary polarization by the magneto-optical disk and orthogonal to the incident polarization direction, and the orthogonal polarized component is amplified with the incident polarized light for detection. Thus, although a signal can be increased if the original incident polarized component is increased, it is necessary to control the light intensity that enters the optical disk to a certain point or below, in order not to erase or overwrite data. In contrast to this, in the conventional-techniques mentioned above, a light beam for interference with signal light is separated in advance, this light beam, which is not focused on the disk, is caused to interfere with the signal light, and the intensity of the light beam for interference used for signal amplification can be made intensified, regardless of the light intensity at the disk surface. Theoretically, this allows an increased signal-to-noise ratio as compared with the noise of an amplifier that converts a photocurrent from a photo detector into a voltage, shot noise produced in a photo detector, or the like, as the intensity is more increased within a permissive range of light intensity.
In Japanese Patent Application Laid-Open Publication No. Hei 5-342678, two light beams are caused to interfere with each other to detect interference intensity. At this time, the optical path length of the light beam that does not reflect off the disk for interference is made variable. This aims to ensure the amplitude of the interference signal. In Japanese Patent Application Laid-Open Publication No. Hei 6-223433, in addition to the interference intensity detection, differential detection is also performed. As a result of this, the intensity component of each of individual light beams not contributing to signals is cancelled, and the noise component of each of the individual light beams is canceled, aiming for an increased signal to noise ratio. For differential detection in this case, a non-polarizing beam splitter is used.
For an exemplary technique that reduces interlayer crosstalk, Japanese Patent Application Laid-Open Publication No. 2001-273640 describes a method for reducing crosstalk by signal processing, by adopting a scheme in which a reflected light beam is divided into three beams to detect a light beam reflected off each of individual layers (a target readout layer and two layers adjacent to the target layer) at different focus positions.